Water is an unavoidable contaminant in fuel. Water can affect components in fuel systems and lead to operational delays and increased maintenance activities. In addition, the propensity for microbiological contamination is directly proportional to the presence of water and the temperature within fuel tanks.
Although water may affect fuel systems of land or water based vehicles, water is a particular problem in aircraft fuel systems. Water may enter aircraft fuel tanks from fuel loaded into the aircraft fuel tanks during refuel (dissolved water) and from air entering the aircraft fuel tanks via its vent system. A vent system to ambient air is normally required to normalise the pressure within the fuel tanks during climb and descent of the aircraft.
Since the solubility of water in fuel decreases with decreasing temperature, during aircraft cruise water dissolution from fuel occurs as the fuel temperature decreases. It forms small droplets of the order of microns. The droplets remain suspended in the fuel and create an almost homogeneous mist or fog-like phenomenon in fuel. The water droplets have a density (around 1000 kg/m3) similar to that of aviation fuel (around 800 kg/m3). The water droplet size and the relative density of the water droplets and the surrounding fuel are key parameters determining the settling rate of the droplets (Stokes' Law). The settling velocity is proportional to the square of the droplet radius. With the droplet size of the order of microns, it takes a long time for the droplets to settle out to the tank bottom. The density difference is small, although significant, but in this case the primary factor determining the slow settling rate of the droplets is their size. The fuel with suspended water droplets is fed to the engine where it is burnt off with the fuel. However, the low concentration of water in suspension means that the rate of water removal from the fuel system is slow.
As the temperature within the fuel tank decreases during the cruise phase of an aircraft flight, the suspended water droplets can turn to ice forming “snow”. The snow takes even longer to sink to the bottom of the fuel tank as the density of the ice (around 900 kg/m3) is even closer to that of the fuel than the water droplets.
In addition, the mist or fog-like phenomenon in fuel tends to be cleared off when a sufficient natural convection current is established in the fuel tank. Drier (unsaturated) fuel carried by the natural convection current from colder tank structures and surfaces re-dissolves the suspended water droplets. The natural convection current carries the saturated fuel to bring it in contact with cold tank surfaces where water dissolution from the fuel causes condensation on cold surfaces. The condensation tends to run down the wall of the fuel tank and collect in pools at the bottom of the tank. Water from these pools can be drained off when the aircraft is on the ground but this is time consuming and costly, leading to a loss of operational efficiency.
U.S. Pat. No. 4,081,373 describes a system in which a cyclonic separator and a water coalescer are connected within a fuel system. Fuel from a fuel tank is fed into the cyclonic separator, which spins the fuel into an intense cyclonic spiral, and centrifugal force separates relatively pure fuel from a fuel-impurity concentrate. The combined cyclonic separator and a water coalescer return “purified” fuel to the fuel tank, and a fuel-impurity mixture is fed to an auxiliary separator. The auxiliary separator returns further “purified” fuel to the fuel tank and a water-solid (impurity) sludge is separated out and periodically drained off. The impurity sludge is exhausted either to the atmosphere or to a collection vessel. Where a collection vessel is used this will still need to be drained when the aircraft is on the ground. In the case of exhausting to the atmosphere, a suitable exhaust system will be required, which adds weight, maintenance costs etc. to the fuel system and could lead to icing problems at the outlet.